A work machine, such as a construction work machine, an agricultural work machine or a forestry work machine, typically includes a prime mover in the form of an internal combustion (IC) engine. The IC engine may either be in the form of a compression ignition engine (i.e., diesel engine) or a spark ignition engine (i.e., gasoline engine). For most heavy work machines, the prime mover is in the form of a diesel engine having better lugging, pull-down and torque characteristics for associated work operations.
The step load response of an IC engine in transient after a load impact is a feature mostly influenced by the engine displacement, the hardware of the engine (e.g., whether it has a standard turbocharger, a turbocharger with waste gate or variable geometry, etc.), and by the software strategy for driving the air and fuel actuators (e.g., exhaust gas recirculation, turbocharger with variable geometry turbine (VGT), fuel injector configuration, etc.) with respect to the requirements of emissions legislation (e.g., visible smoke, nitrous oxides (NOx), etc.), noise or vibrations. The load impact may be the result of a drivetrain load (e.g., an implement towed behind the work machine) or an external load (e.g., an auxiliary hydraulic load such as a front end loader, backhoe attachment, etc.)
Engine systems as a whole react in a linear manner during the application of a transient load. Initially, the load is applied to the drive shaft of the IC engine. The IC engine speed decreases when the load increases. The engine speed drop is influenced by whether the governor is isochronous or has a speed droop. The air flow is increased to provide additional air to the IC engine by modifying the air actuators. A time delay is necessary to achieve the new air flow set point. The fuel injection quantity, which is nearly immediate, is increased with respect to both the smoke limit and maximum allowable fuel quantity. The engine then recovers to the engine speed set point. The parameters associated with an engine step load response in transient after a load impact are the speed drop and the time to recover to the engine set point.
An IC engine may be coupled with an IVT which provides continuous variable output speed from 0 to maximum in a stepless fashion. An IVT typically includes hydrostatic and mechanical gearing components. The hydrostatic components convert rotating shaft power to hydraulic flow and vice versa. The power flow through an IVT can be through the hydrostatic components only, through the mechanical components only, or through a combination of both depending on the design and output speed.
One example of an IVT for use in a work machine is a hydromechanical transmission which includes a hydraulic module coupled with a planetary gear set. Another example of an IVT for a work machine is a hydrostatic transmission which includes a hydraulic module coupled with a gear set.
A work machine including an IVT may be prone to loss of traction control and wheel slip when the IVT ratio changes to match load conditions. The IVT controller senses engine speed and deepens the IVT ratio as engine speed decreases under load. When at low ground speeds, the amount of power required for the work machine is a low percentage of what the engine can generate, so the engine may not lug down when the output torque from the engine increases. The operator will then not be aware that the torque at the wheels is increasing. In this case, the drive wheels can lose traction and spin out without notice. This is undesirable for certain operations.
What is needed in the art is a work machine configured with an IVT which is not prone to losing traction in low ground speed conditions.